BACKGROUND: The objectives of this study were optimization of silver nanoparticle synthesis using biotransformations by Lactobacillus casei subsp. casei, and studying the location of nanoparticles synthesis in this microorganism. RESULTS: The presence of AgNO3 (0.1 mmol L-1) in the culture as the enzyme inducer, and glucose (56 mmol L-1) as the electron donor in the reaction mixture had positive effects on nanoparticle production. By gradually increasing the concentration of AgNO3 (as the substrate) to 6 mmol L-1, nanoparticle production was increased. By increasing biomass, nanoparticles production was also increased. Biosynthesized silver nanoparticles were almost spherical, single (2550 nm) or in aggregates (100 nm), attached to the surface of biomass or were inside and outside of the cells. CONCLUSION: The present study demonstrated the bioreductive synthesis of silver nanoparticles using L. casei subsp. casei at room temperature. In this research, and due to experience in optimization of biotransformation reactions, the reaction conditions were successfully optimized to increase the yield of nanoparticles production and productivity of this biosynthetic approach. Copyright (c) 2012 Society of Chemical Industry